If the singers are on wedge mixes I think it appropriate to have the wedge sends on while experimenting with the FOH mix. The amount of "stuff" that wedges contribute to the over all wash of stage SPL can have a significant contribution to FOH mix/tonality issues.
These are the things that "virtual soundcheck" are for!
Indeed. I've mixed bands where they insisted on turning up the monitors to the point where you could just cut the mains and it didn't make much difference at FOH
IEM's or properly controlled monitor levels make a huge difference. Lots of great comments in this thread, but I'll add two things:
First is a graphic I recently stumbled across in another thread. Great quick reference for go to frequencies based on common descriptions. (attached)
Second is always remember the source has the greatest impact on the sound. Next comes the acoustic space, followed by mic placement, mic choice, PA mains (and placement), and finally what you do at the desk. The source and the acoustic space are usually out of your control, but what mic you use and where you place it are powerful tools you can control.